Simple example:
A cavalry UG starts just in front of an enemy SuG. In the movement phase the cavalry advance 5 BW. Under the pushing back enemy SuGs rule, the SuG gets pushed back to 1BW away from the cavalry. The SuG has been pushed back over 5 BW. The maximum movement distance for the SuG is 4 BW, so the player owning the cavalry rolls a KaB test on the SuG.
Note: The SuG might also exceed its maximum movement distance by being pushed back through a friendly TuG.
Hope that helps.
Richard